This invention relates to a bin, hopper or ship's cargo hold gate used in controlling the flow of a particulate material out of the bottom of the bin, hopper, or cargo hold.
The cargo spaces of ships adapted to convey particulate solids in bulk, generally known as bulk carriers, comprise a series of cargo holds which are in many ways similar to bulk bins or hoppers used in other applications to contain similar particulate solids. In this context, by "particulate solids" is meant any particulate solid material which is normally conveyed in bulk, in high volume; typical examples are crushed coal, many mineral ores including powdered sulphur, crushed rock, salt, fertiliser, saltpetre and various types of grain. These materials are well adapted to being moved about by continuous feed machinery, typically including the use of continuous belt conveyors, bucket elevators, and the like.
Although a bin or hopper can be emptied from the bottom with a gate mechanism relatively easily, in a bulk carrier emptying a cargo hold poses different problems. In a so-called "self unloading" bulk carrier, a discharge gate system is located in the bottom of each hold, which serves to control transfer of the particulate solids onto a first conveyor means located in a tunnel under the cargo holds. The conveyor moves the received solids along the tunnel, beneath the holds, to an elevator means which is generally at one end of the bulk cargo space, for example in the hull forecastle. The elevator moves the particulate solids essentially vertically, to a point from which they can be discharged from the ship, generally carried by a second conveyor means. In some self unloading bulk carriers the cargo compartments include two or three laterally spaced gate systems, together with the required tunnels and conveyers.
In many self unloading bulk carriers the discharge gate system comprises a row of centre opening gates, often described as "basket gates", generally located so that the axis of opening is along the length of the conveyor beneath the gate. The bottom of the hold is tapered downwardly to the (or each) row of gates to facilitate solids flow. The length of the gate opening can be up to 3 meters, and the open gate aperture can also be up to 2 meters wide. Each basket gate mechanism typically includes two opposed gate segments, and a hydraulic cylinder system to move the segments to open and to close the gate aperture; it is also possible to use linear electrically powered actuators or pneumatic cylinders instead of hydraulic devices. A feature common to all basket gates currently in use is that a gear mechanism is used to ensure that the gate segments move together and provide equal opening about the centre line of the gate opening. The gate has to be a substantial structure, as it has to support the load imposed by the cargo when closed, which also means that significant force can be required to open it, or to close it.
Although gate openings have increased in length and width, the construction of the so-called basket gate has hardly changed. Each gate is mounted between frames which support the ends of the gate segments, the gear mechanisms used to coordinate gate segment movement, and the hydraulic cylinders used to move them; other than at the ends of a row of basket gates, each frame generally supports the ends of two adjacent gates. The frames and mechanisms between each gate are supported by structures in the bottom of each cargo hold, and are protected by a covering structure, known as a hog back. But as the basket gate has grown larger, the space required for the gear mechanisms and the hydraulic systems has increased, thus requiring a larger hog back, to the detriment of the cargo holding space. Further, in the known basket gate, and in the known centre split roller track gate, the hydraulic system is arranged to act onto either the gear mechanisms, or the ends of, the gate segments directly, with the result that for each gate at least two, and often four, hydraulic cylinders are required, which both increases first cost and hydraulic installation complexity (especially if a remote control system is used) and requires significant maintenance.
A need therefore exists for a simpler, less complex, and more compact basket gate, which will occupy a smaller space within the cargo holds of a bulk carrier. Such a mechanism will have applicability more generally in other bulk holding bins and hoppers.